Disease and Immunity 101 



and lost by Idleness, the child is what the parents 

 were at the time of reproducing, and not what 

 they were at some earlier date or will be at some 

 later time. 



When we examine cases of natural immunity 

 to specific diseases we find that it exists only in 

 those animals and plants which have lived for 

 several generations in contact with the disease 

 producing parasite. Zebu cattle from India are 

 said to be immune to the Texas fever, but this 

 immunity is to attacks by the tick rather than 

 to the disease as such. This is not the kind of 

 immunity we have under consideration. What 

 we want to know is why the range cattle of 

 Texas, when bitten by the Texas ticks, do not 

 suffer from the fever as do northern cattle when 

 taken to Texas. Also, why the West Indian 

 negro does not get yellow fever when bitten by 

 the proper mosquito ; why the Balkan Ziginars 

 are not stricken with typhus fever when bitten 

 by the Balkan lice; why native American vines 

 resist the American phylloxera. 



In any epidemic, a considerable proportion of 

 the population appear to escape attacks, and of 

 those attacked, a considerable proportion re- 



